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On a “cholesteric” phase in disc-like mesogens
Volume 79A, number 2,3
PHYSICS LETTERS
29 September 1980
ON A “CHOLESTERIC” PHASE IN DISC-LIKE MESOGENS C. DESTRADE and NGUYEN HUU TINH Centre de Recherche Paul Pascal, 33405 Talence, France
and J. MALTHETE and J. JACQUES College de France, Laboratoire de Chimie Organique des Hormones, 75231 Paris Cedex, France Received 12 June 1980
Several chiral disc-like compounds are described. They exhibit nonchiral D columnar phases. But it is shown that, for example, chiral triphenylenes are able to twist the ND nematic phase of a hexabenzoate of triphenylene. That is, probably, the first evidence of a “cholesteric” phase in disc-like mesogens.
The recent discovery of a ND nematic phase with disc-like mesogens [1,2] raises the question of the possible existence of a corresponding cholesteric phase, associated with the apparition of a chirality in this medium. We have pointed out [1] that the introduction of a
where R represents the chiral 3-methyl nonanoyl group. The (+) or (—) 3-methyl nonanoic acid was prepared following Prout et a!. [9], from (+) or (—) 2-octanol with some convenient modifications. The corresponding chloride (prepared with SOC12)was first condensed
small amount of a chiral substance in a hexabenzoate of triphenylene does not induce any observable mothfication in the optical texture of the ND nematic phase. Nevertheless we have thought it would be interesting to prepare optically active substances with disc-like shape and to look for the consequences of the molecular chirality on these mesomorphic phases. In this note we describe the enantiomeric esters corresponding to the structures 1, 2 and 3 with wellknown mesogenic cores [2—81:
with hexahydroxybenzene [101, then with 2,3,6,7,10, 11 hexahydroxy triphenylene [51and finally with 2,3 7,8,12,13 hexahydroxy truxene [21, to give respectively: 1 ([a]~8 = +6.8°and —6.7°,CHC13, C 1), 2 ([~]~8 = + 12.4°and —11.6°,CHCI3, C~1)and 3 ([~1578 = + 12.0°and —11.9°,CHCI3, C 0.5). The corresponding heats (kcal/mole) and temperatures of the transitions (°C),measured with DSC2 Perkin—Elmer, are given in table 1; the two enantiomers of 1, 2 and 3 and the racemic mixture exhibit the same transitions. The same behaviour is observed with
RO OR RO
OR
OR OR OR
RO
RO~~cR OR
RO OR OR
R = —CO—CH2—CHC6 H13 CH3
RO RO
Q ~
OR OR
n,
some mixtures of variable optical purity: the enantiomers give a solid solution with minimum. Observations with a heating stage microscope (Mettler FP5) show the existence of two mesomorphic phases for 2 and 3. Optical texture observations and binary phase diagram studies on some corresponding achiral esters (R = COCnH2n+i) show that the lower temperature phase is a rectangular arrangement of a column of discs (Dr columnar phase [8,11,121) while the highest temperature one is a hexagonal arrangement 189
Volume 79A, number 2,3
PHYSICS LETTERS
Table 1 C, C 1, C2 arc crystal phases, Dr is the rectangular columnar phase [8,13,14], Dh is the hexagonal columnar phase [8,13, 14]. ——.
2(+) 2(--) 3(~i-)b)
29 September 1980
f”~ ~ ______
~-.-..—
C1—C2
C2—I
C~Dr
Dr~Dh
Dh°l
59(5.98) 59(5.90) 1~h Dr 103
—. a) 64.5(0.016) Dh I c)
97.5 (1.01) 97.5(1.06)
230
116 235 a) Not observed by calorimetry. 59°Cby microscopic observations. b) Measured by microscopic observation. No crystailisation, Dr exists at room temperature. c) With some decomposition of the sample.
____________________________________________________
phase [8,11,12]). Therefore the nature of these phases is not affected by the chiral substituent. But contact between the 2,3,6,7,10,11 hexa-nheptyloxybenzoate of triphenylene [131, C 7OHBT, (Dh columnar
R=nC7Hi5O-~©CO—, C 170 ND
2~j
and derivative 1, 2, 3 (+) or (—) exhibits at high tem-
perature a domain with very typical cholesteric textures [14] : oily streaks (fig. Ia), finger prints or polygonal textures (fig. ib). Although the complete binary phase diagram has not yet been obtained, the study of a 50/50 (weight) mixture gives interesting information and confirms the “cholesteric” twist of the ND nematic phase. For example, with the 2 derivative (+) or (—), on heating the solid is observed up to 63°C(fig. 2a). Then, a fluid phase appears. From an optical texture point of view, this phase is in every respect similar to the ND nematic phase (fig. 2b). It is only near 100°Cthat the cholesteric textures begin to be observed: cell textures (fig. 2c), then, at higher temperature (from 120°to 144°C), oily streaks, a bundle of oily streaks and a polygonal texture (such as in figs. la,b). We have not seen any fan-like texture. When 2(+) in C7 OHBT (50/50) and 2(—) in C7OHBT (50/50) are mixed by contact, a ND nematic phase is observed between the two “chol190
Fig. 1. Optical textures observed at contact between C7OHBT and derivative 2(+) at 160°C.
esteric” domains as with classical liquid crystals [15]. Other observations should be pointed out. In the cholesteric range of temperature, a continuous change in the medium from red or yellow-red to a dark-blue colour is observed. In fact in some preparations large blue areas without any defects are observed! If the typical texture of this new “cholesteric” phase has been clearly identified ~‘, an important difference with the classical cholesteric phase has to be noted: a great amount of chiral disc-like molecules seems to be necessary to twist, to an observable extent, the ND nematic phase. Furthermore the twist has been only observed at high temperature “after” a nematic texture phase; we do not pretend, of course, that we have observed a ND -~N~transition! But merely that we have a continuous and important decrease of the ~ We propose to call it N~or NDtW.
Volume 79A, number 2,3
PHYSICS LETTERS
29 September 1980
~
//
~
/
_______________
/
__~c~
z1~~ /
U~U U n U
z~
U
//
/
P/2
~
.../ /
a \
I I•’I ~I
ui__li
i~ I
f:~~ L
(.
°“-~•••-•/•~:-~
~
Fig. 3. Structure of the cholesteric phase. (a) Rod-like liquid crystals (N*). (b) Disc-like liquid crystals (Ni). —° is the optical axis. p12: half of the cholesteric pitch. Fig. 2. Optical textures observed from a 50/50 mixture of C 7OHBT and derivative 2(+). (a) Solid phase at 50°C. (b) optical texture at 90°C.(C) N~cholesteric phase at 130°C.
with temperature. The existence of the blue colour area (it is always observed in any preparation) poses an interesting problem. Now we are preparing some chiral hexabenzoates of triphenylene with the hope that they present a pure N~phase. “cholesteric” pitch
One surely wonders what the structure of this phase compared with the classic N* one. In fact, we can say [8] that the important difference between classical (rod-like) and disc-like liquid crystals is that, in the first case, the molecules are parallel to the optical axis, while in the other the flat—round molecules are perpendicular to it. It is obvious that the ND phase corresponds to the nematic phase N. Now one can Unis
191
Volume 79A, number 2,3 derstand
PHYSICS LETTERS
easily the structure of the N~phase we pro-
pose (figs. 3a, b).
In conclusion, up to now and in a very short time, several columnar phases: Dh
0, Dhd, Drd,
D~[8] have
been proved to exist. Then, the existence of a ND nematic phase was discovered [1]. Here, we present evidence of a twisted N~nematic phase. In another letter, we have shown a phenomenon very similar to a reentrant nematic phase in some truxene derivatives (what we call: “an inverted nematic—columnar phase sequence” [2]). These results suggest the existence of other twisted phases such ~s D~’in analogy with chiral tilted smectic phases. We thank Professor H. Gasparoux and J. Prost for stimulating discussions and helpful suggestions. References [1] Nguyen Huu Tinh, C. Destrade and Fl. Gasparoux, Phys. Lett. 72A (1979) 251. [2] C. Destrade, J. Malthete, Nguyen Huu Tinh and ~ Gasparoux, Phys. Lett. 78A (1980) 82. [3] S. Chandrasekhar, B.K. Sadashiva and K.A. Suresh, Pramana9 (1977) 471.
192
29 September 1980
[4] J. Billard, iC. Dubois, Nguyen FIuu Tinh and A. Zann, Nouv. J. de Chim. 2 (1978) 535. [5] C. Destrade, M.C. Mondon and J. Maithete, J. de Phys. Colloq. 40 (1979) C3-17. [6] C. Destrade, M.C. Mondon-Bernaud and Nguyen Huu Tinh, Mol. Cryst. Liq. Cryst. Lett. 49 (1979) 169. [7] Nguyen Huu Tinh, M.C. Bernaud, G. Sigaud and C.
Destrade, 3rd Liq. cryst. Conf. of Socialist countries (Budapest, 1979); and submitted to Mol. Cryst. Liq. [8] Cryst. C. Destrade, M.C. Bernaud, H. Gasparoux, A.M. Levelut and Nguyen Huu Tinh, Proc. Intern. Liq. cryst. Conf. (Bangalore, 1979). [9] ES. Prout, D.E. Dikhson and R.J. Klimkowski, J. Org. Chem. 24 (1959) 826.
[10] A.J. Fatiadi and W.F. Sager, Org. Synth. Coll. Vol. V, p. 595. [11] A.M. Levelut, Proc. Intern. Liq. cryst. Conf. (Bangalore, 1979). [121 C. Destrade, Nguyen Huu Tinh, H. Gasparoux, J. Malthete and A.M. Levelut, 3rd Liq. cryst. Conf. of Socialist countries (Budapest, 1979). [13] Nguyen Huu Tinh, C. Destrade and H. Gasparoux, 8th Intern. Liq. cryst. Conf. (Kyoto, 1980). [14] D. Demus and L. Richter, Textures of liquid crystals (Verlag Chemie, Weinheim, New York, 1978). [15] M. Leclercq, J. Bfflaxd and J. Jacques, Mol. Cryst. Liq. Cryst. 8 (1979) 367.
PHYSICS LETTERS
29 September 1980
ON A “CHOLESTERIC” PHASE IN DISC-LIKE MESOGENS C. DESTRADE and NGUYEN HUU TINH Centre de Recherche Paul Pascal, 33405 Talence, France
and J. MALTHETE and J. JACQUES College de France, Laboratoire de Chimie Organique des Hormones, 75231 Paris Cedex, France Received 12 June 1980
Several chiral disc-like compounds are described. They exhibit nonchiral D columnar phases. But it is shown that, for example, chiral triphenylenes are able to twist the ND nematic phase of a hexabenzoate of triphenylene. That is, probably, the first evidence of a “cholesteric” phase in disc-like mesogens.
The recent discovery of a ND nematic phase with disc-like mesogens [1,2] raises the question of the possible existence of a corresponding cholesteric phase, associated with the apparition of a chirality in this medium. We have pointed out [1] that the introduction of a
where R represents the chiral 3-methyl nonanoyl group. The (+) or (—) 3-methyl nonanoic acid was prepared following Prout et a!. [9], from (+) or (—) 2-octanol with some convenient modifications. The corresponding chloride (prepared with SOC12)was first condensed
small amount of a chiral substance in a hexabenzoate of triphenylene does not induce any observable mothfication in the optical texture of the ND nematic phase. Nevertheless we have thought it would be interesting to prepare optically active substances with disc-like shape and to look for the consequences of the molecular chirality on these mesomorphic phases. In this note we describe the enantiomeric esters corresponding to the structures 1, 2 and 3 with wellknown mesogenic cores [2—81:
with hexahydroxybenzene [101, then with 2,3,6,7,10, 11 hexahydroxy triphenylene [51and finally with 2,3 7,8,12,13 hexahydroxy truxene [21, to give respectively: 1 ([a]~8 = +6.8°and —6.7°,CHC13, C 1), 2 ([~]~8 = + 12.4°and —11.6°,CHCI3, C~1)and 3 ([~1578 = + 12.0°and —11.9°,CHCI3, C 0.5). The corresponding heats (kcal/mole) and temperatures of the transitions (°C),measured with DSC2 Perkin—Elmer, are given in table 1; the two enantiomers of 1, 2 and 3 and the racemic mixture exhibit the same transitions. The same behaviour is observed with
RO OR RO
OR
OR OR OR
RO
RO~~cR OR
RO OR OR
R = —CO—CH2—CHC6 H13 CH3
RO RO
Q ~
OR OR
n,
some mixtures of variable optical purity: the enantiomers give a solid solution with minimum. Observations with a heating stage microscope (Mettler FP5) show the existence of two mesomorphic phases for 2 and 3. Optical texture observations and binary phase diagram studies on some corresponding achiral esters (R = COCnH2n+i) show that the lower temperature phase is a rectangular arrangement of a column of discs (Dr columnar phase [8,11,121) while the highest temperature one is a hexagonal arrangement 189
Volume 79A, number 2,3
PHYSICS LETTERS
Table 1 C, C 1, C2 arc crystal phases, Dr is the rectangular columnar phase [8,13,14], Dh is the hexagonal columnar phase [8,13, 14]. ——.
2(+) 2(--) 3(~i-)b)
29 September 1980
f”~ ~ ______
~-.-..—
C1—C2
C2—I
C~Dr
Dr~Dh
Dh°l
59(5.98) 59(5.90) 1~h Dr 103
—. a) 64.5(0.016) Dh I c)
97.5 (1.01) 97.5(1.06)
230
116 235 a) Not observed by calorimetry. 59°Cby microscopic observations. b) Measured by microscopic observation. No crystailisation, Dr exists at room temperature. c) With some decomposition of the sample.
____________________________________________________
phase [8,11,12]). Therefore the nature of these phases is not affected by the chiral substituent. But contact between the 2,3,6,7,10,11 hexa-nheptyloxybenzoate of triphenylene [131, C 7OHBT, (Dh columnar
R=nC7Hi5O-~©CO—, C 170 ND
2~j
and derivative 1, 2, 3 (+) or (—) exhibits at high tem-
perature a domain with very typical cholesteric textures [14] : oily streaks (fig. Ia), finger prints or polygonal textures (fig. ib). Although the complete binary phase diagram has not yet been obtained, the study of a 50/50 (weight) mixture gives interesting information and confirms the “cholesteric” twist of the ND nematic phase. For example, with the 2 derivative (+) or (—), on heating the solid is observed up to 63°C(fig. 2a). Then, a fluid phase appears. From an optical texture point of view, this phase is in every respect similar to the ND nematic phase (fig. 2b). It is only near 100°Cthat the cholesteric textures begin to be observed: cell textures (fig. 2c), then, at higher temperature (from 120°to 144°C), oily streaks, a bundle of oily streaks and a polygonal texture (such as in figs. la,b). We have not seen any fan-like texture. When 2(+) in C7 OHBT (50/50) and 2(—) in C7OHBT (50/50) are mixed by contact, a ND nematic phase is observed between the two “chol190
Fig. 1. Optical textures observed at contact between C7OHBT and derivative 2(+) at 160°C.
esteric” domains as with classical liquid crystals [15]. Other observations should be pointed out. In the cholesteric range of temperature, a continuous change in the medium from red or yellow-red to a dark-blue colour is observed. In fact in some preparations large blue areas without any defects are observed! If the typical texture of this new “cholesteric” phase has been clearly identified ~‘, an important difference with the classical cholesteric phase has to be noted: a great amount of chiral disc-like molecules seems to be necessary to twist, to an observable extent, the ND nematic phase. Furthermore the twist has been only observed at high temperature “after” a nematic texture phase; we do not pretend, of course, that we have observed a ND -~N~transition! But merely that we have a continuous and important decrease of the ~ We propose to call it N~or NDtW.
Volume 79A, number 2,3
PHYSICS LETTERS
29 September 1980
~
//
~
/
_______________
/
__~c~
z1~~ /
U~U U n U
z~
U
//
/
P/2
~
.../ /
a \
I I•’I ~I
ui__li
i~ I
f:~~ L
(.
°“-~•••-•/•~:-~
~
Fig. 3. Structure of the cholesteric phase. (a) Rod-like liquid crystals (N*). (b) Disc-like liquid crystals (Ni). —° is the optical axis. p12: half of the cholesteric pitch. Fig. 2. Optical textures observed from a 50/50 mixture of C 7OHBT and derivative 2(+). (a) Solid phase at 50°C. (b) optical texture at 90°C.(C) N~cholesteric phase at 130°C.
with temperature. The existence of the blue colour area (it is always observed in any preparation) poses an interesting problem. Now we are preparing some chiral hexabenzoates of triphenylene with the hope that they present a pure N~phase. “cholesteric” pitch
One surely wonders what the structure of this phase compared with the classic N* one. In fact, we can say [8] that the important difference between classical (rod-like) and disc-like liquid crystals is that, in the first case, the molecules are parallel to the optical axis, while in the other the flat—round molecules are perpendicular to it. It is obvious that the ND phase corresponds to the nematic phase N. Now one can Unis
191
Volume 79A, number 2,3 derstand
PHYSICS LETTERS
easily the structure of the N~phase we pro-
pose (figs. 3a, b).
In conclusion, up to now and in a very short time, several columnar phases: Dh
0, Dhd, Drd,
D~[8] have
been proved to exist. Then, the existence of a ND nematic phase was discovered [1]. Here, we present evidence of a twisted N~nematic phase. In another letter, we have shown a phenomenon very similar to a reentrant nematic phase in some truxene derivatives (what we call: “an inverted nematic—columnar phase sequence” [2]). These results suggest the existence of other twisted phases such ~s D~’in analogy with chiral tilted smectic phases. We thank Professor H. Gasparoux and J. Prost for stimulating discussions and helpful suggestions. References [1] Nguyen Huu Tinh, C. Destrade and Fl. Gasparoux, Phys. Lett. 72A (1979) 251. [2] C. Destrade, J. Malthete, Nguyen Huu Tinh and ~ Gasparoux, Phys. Lett. 78A (1980) 82. [3] S. Chandrasekhar, B.K. Sadashiva and K.A. Suresh, Pramana9 (1977) 471.
192
29 September 1980
[4] J. Billard, iC. Dubois, Nguyen FIuu Tinh and A. Zann, Nouv. J. de Chim. 2 (1978) 535. [5] C. Destrade, M.C. Mondon and J. Maithete, J. de Phys. Colloq. 40 (1979) C3-17. [6] C. Destrade, M.C. Mondon-Bernaud and Nguyen Huu Tinh, Mol. Cryst. Liq. Cryst. Lett. 49 (1979) 169. [7] Nguyen Huu Tinh, M.C. Bernaud, G. Sigaud and C.
Destrade, 3rd Liq. cryst. Conf. of Socialist countries (Budapest, 1979); and submitted to Mol. Cryst. Liq. [8] Cryst. C. Destrade, M.C. Bernaud, H. Gasparoux, A.M. Levelut and Nguyen Huu Tinh, Proc. Intern. Liq. cryst. Conf. (Bangalore, 1979). [9] ES. Prout, D.E. Dikhson and R.J. Klimkowski, J. Org. Chem. 24 (1959) 826.
[10] A.J. Fatiadi and W.F. Sager, Org. Synth. Coll. Vol. V, p. 595. [11] A.M. Levelut, Proc. Intern. Liq. cryst. Conf. (Bangalore, 1979). [121 C. Destrade, Nguyen Huu Tinh, H. Gasparoux, J. Malthete and A.M. Levelut, 3rd Liq. cryst. Conf. of Socialist countries (Budapest, 1979). [13] Nguyen Huu Tinh, C. Destrade and H. Gasparoux, 8th Intern. Liq. cryst. Conf. (Kyoto, 1980). [14] D. Demus and L. Richter, Textures of liquid crystals (Verlag Chemie, Weinheim, New York, 1978). [15] M. Leclercq, J. Bfflaxd and J. Jacques, Mol. Cryst. Liq. Cryst. 8 (1979) 367.